Rechargeable lithium battery technology has become increasingly important in recent years because it is providing new, lightweight, high energy density batteries for powering applications in the rapidly growing electronics industry. These batteries are also of interest because of their possible application in electric vehicles and hybrid electric vehicles. State-of-the-art rechargeable lithium batteries are known as "lithium-ion" batteries because during charge and discharge, lithium ions are shuttled between two host electrode structures with a concomitant reduction and oxidation of the host electrodes. The best known lithium-ion cell is a 3.5 V Li.sub.x C.sub.6 /Li.sub.1-x CoO.sub.2 cell, in which lithium is extracted from a layered LiCoO.sub.2 structure (positive electrode or cathode) during charge and inserted into a carbonaceous structure (negative electrode or anode), typically graphite or a "hard" or pyrolyzed carbon. Lithiated carbons can approach and reach the potential of metallic lithium at the top of the charge cycle. Therefore, these negative electrodes or anodes are highly reactive materials, particularly in the presence of a highly oxidizing Li.sub.1-x CoO.sub.2 positive electrode and a flammable organic electrolyte. There is, therefore, a concern about the safety of charged lithium-ion cells; sophisticated electronic circuitry has to be incorporated into each cell to protect them from overcharge and abuse. This invention addresses the need to find alternative negative electrode materials to carbon.
The spinel, Li.sub.4 Ti.sub.5 O.sub.12, is an attractive alternative negative electrode material to carbon. Three lithium ions can be inserted into the structure according to the reaction: EQU 3Li+Li.sub.4 Ti.sub.5 O.sub.12.fwdarw.Li.sub.7 Ti.sub.5 O.sub.12
This reaction occurs at approximately 1.5 V vs. metallic lithium, thereby providing a relatively safe electrode system compared to carbon. However, safety is gained at the expense of cell voltage and energy density. A further limitation is that Li.sub.4 Ti.sub.5 O.sub.12 provides a relatively low theoretical capacity (175 mAh/g) compared to lithiated graphite (LiC.sub.6, 372 mAh/g). Nevertheless, despite these limitations, cells with lithium-titanium-oxide negative electrodes can still be coupled to high voltage (4 V) positive electrode materials, such as the layered oxides LiCoO.sub.2, LiNiO.sub.2, LiCo.sub.1-x Ni.sub.x O.sub.2, and spinel oxides, for example, LiMn.sub.2 O.sub.4 to provide cells with an operating voltage of between 2.4 and 2.2 V. It is anticipated that these cells will become increasingly attractive from at safety standpoint, particularly as the voltage requirement for powering semiconducting devices decreases in time. For example, a 2.4 V lithium-ion cell can be constructed by coupling two spinel electrodes: EQU Li.sub.4+x Ti.sub.5 O.sub.12 +3 Li.sub.1-x/3 Mn.sub.2 O.sub.4 &lt;- - -&gt;Li.sub.4 Ti.sub.5 O.sub.12 +3 LiMn.sub.2 O.sub.4
From a structural viewpoint, Li.sub.4 T.sub.15 O.sub.12 is an example of an almost ideal host electrode for a lithium-ion cell. Lithium insertion into the cubic Li.sub.4 T.sub.15 O.sub.12 spinel structure occurs with virtually no change in the lattice parameter (8.36 .ANG.); the unit cell expands and contracts isotropically during lithium insertion and extraction, thereby providing an extremely stable electrode structure; it can undergo many hundreds of cycles without structural disintegration. Moreover, lithium insertion causes a first-order displacement of the tetrahedrally-coordinated lithium ions in the Li.sub.4 Ti.sub.5 O.sub.12 spinel structure into octahedral sites to generate the ordered rock salt phase Li.sub.7 Ti.sub.5 O.sub.12. The insertion (and extraction) of lithium is thus a two-phase reaction which provides a constant voltage response (at approximately 1.5 V). Furthermore, the voltage of a Li/Li.sub.4+x Ti.sub.5 O,.sub.12 cell changes abruptly at the end of discharge and charge. Thus, a Li.sub.4+x Ti.sub.5 O.sub.12 spinel electrode provides very sharp end-of-charge and end-of-discharge indicators which is useful for controlling cell operation and preventing overcharge and overdischarge.
A major disadvantage of a Li.sub.4 Ti.sub.5 O.sub.12 spinel electrode is that all the titanium ions in the structure are tetravalent; the material is thus an insulator, with negligible electronic conductivity--it is white in color. Good insertion electrodes should have both good ionic conductivity to allow rapid lithium-ion diffusion within the host and good electronic conductivity to transfer electrons from the host structure to the external circuit during charge and discharge. To overcome poor electronic conductivity, it is customary to add an electronic current collector, such as carbon, to metal oxide host electrodes. Thus, throughout the discharge and charge processes, the two-phase Li.sub.4+x Ti.sub.5 O.sub.12 electrode will consist of an insulating Li.sub.4 Ti.sub.5 O.sub.12 spinel phase (in which the titanium ions are all tetravalent) and a mixed-valent, electronically-conducting rock salt phase Li.sub.7 Ti.sub.5 O.sub.12, in which the mean oxidation state of the titanium ions is 3.4 (i.e., 60% Ti.sup.3+ and 40% Ti.sup.4+). Thus, when lithium is extracted from Li.sub.7 Ti.sub.5 O.sub.12, the insulating phase Li.sub.4 Ti.sub.5 O.sub.12 will be formed at the surface of the electrode particles. The insulating properties of Li.sub.4 Ti.sub.5 O.sub.12 will inhibit electronic conductivity at the surface of the particles, thus reducing the rate of electron transfer and the capability of the cell to pass current.